This invention relates to the joining of structures and, more particularly, to the joining achieved by metallic interdiffusion using a non-deforming ceramic-foam bonding element.
Complex structures require that smaller individual components and subassembly structures be joined together. A wide variety of joining techniques exist, including mechanical joining and metallurgical joining. Mechanical joining includes techniques such as mechanical interlocks, mechanical fasteners, and adhesive bonding. Metallurgical joining includes techniques such as soldering, brazing, welding, and diffusion bonding. The selection of a joining technique to be used in a particular circumstance is made based a number of factors including the materials to be joined, structural requirements, service requirements, and permanency requirements.
Metallurgical joining techniques have advantages over mechanical joining techniques in some applications. The metallurgical joining techniques have the potential for achieving a permanent joint of equal strength to the base metals and with essentially no added weight or mechanical complexity. Brazing and soldering are limited in the temperature capability of the final joined structure. Diffusion bonding is an attractive possibility that is applied in some limited circumstances, but many combinations of materials cannot be interdiffused in the solid state to achieve the diffusion bond.
Liquid phase joining techniques such as welding may be used. However, some materials are chemically and physically incompatible so that they cannot be joined by high-temperature joining techniques such as welding. Welding also may adversely affect the metallurgical microstructures in the volumes adjacent to the weld, and result in a volume around the weld that cannot be properly heat treated or otherwise processed to have optimal properties. Locally melting the areas to be joined and then forcing the areas together, as in induction-heated, inertia, and friction welding, has some applications but is limited due to geometrical constraints and the inability to hold tight tolerances during the period that the local areas are melted.
The present inventors are concerned with producing joined structures for use in gas turbine engines. Tight tolerances and alignments must be maintained in metallurgical joints for these applications, and in many cases the geometries of the structures to be joined are not amenable to the use of conventional metallurgical joining techniques. There is therefore a need for an improved joining approach to be used in these and other applications. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for joining structures with a metallurgical bond. The metals facing the joint are melted to achieve rapid, thorough interdiffusion and bonding at the joint. Even though the joint is melted, tolerances and structural alignments are preserved. The present approach may be used in a variety of applications and has wide flexibility to join structures of differing metallic compositions.
A joining method comprises the steps of furnishing a first structure comprising a first-structure metal, and providing a bonding element having a first portion formed of a first open-cell solid ceramic foam. The first open-cell solid ceramic foam comprises ceramic first-portion cell walls, and an intracellular first-portion volume therebetween. The intracellular first-portion volume is filled with a bonding-element first-portion metal. Preferably, the intracellular first-portion volume comprises at least about 20 percent by volume of the first portion of the bonding element.
The method further includes joining the first structure to the bonding element. The step of joining includes the substeps of contacting together the first structure and the first portion of the bonding element at a first bonding temperature greater than a solidus temperature of at least one of the first-structure metal and the bonding-element first-portion metal, and simultaneously interdiffusing the first-structure metal and the bonding-element first-portion metal to form a joined structure. The joined structure is thereafter cooled to room temperature. Preferably, the first bonding temperature is greater than the solidus temperatures of both the first-structure metal and the bonding-element first-portion metal, so that the metal on both sides of the joint is melted.
In the application of particular interest, the first-structure metal and the bonding-element first-portion metal are each nickel-base superalloys. Most preferably, the first-structure metal and the bonding-element first-portion metal are of substantially the same composition.
The joining method may also be extended to joining a second structure, comprising a second-structure metal, to the first structure. The method includes providing the bonding element having a second portion formed of a second open-cell solid ceramic foam comprising ceramic second-portion cell walls, and an intracellular second-portion volume therebetween. The intracellular second-portion volume is filled with a bonding-element second-portion metal. The method includes joining the second structure to the bonding element. The step of joining includes the substeps of contacting together the second structure and the second portion of the bonding element at a second bonding temperature greater than a solidus temperature of at least one of the second-structure metal and the bonding-element second-portion metal, and simultaneously interdiffusing the first-structure metal and the bonding-element first-portion metal to form a joined structure. Thereafter the joined structure is cooled to room temperature. The two stages of joining the first structure to the bonding element, and joining the second structure to the bonding element, may be accomplished simultaneously or sequentially.
The present approach is based on the use of a ceramic foam bonding element. The ceramic foam is formed of the internally continuous network of ceramic cell walls that serves as a skeleton, with the internally continuous intracellular volume between the ceramic cell walls. The intracellular volume is filled with a-metal. The ceramic cell walls retain their strength even when the metal is melted. Consequently, when the bonding element is pressed against a surface of the structure to which it is to be joined and the intracellular metal is melted, the bonding element retains its shape because of the ceramic skeleton. The intracellular metal that faces the structure to be bonded is interdiffused with the metal of the structure, leading to a strong metallurgical bond.
The ceramic foam bonding element is prepared by providing a piece of a sacrificial ceramic having the shape of the bonding element, and contacting (preferably by immersion) the piece of the sacrificial ceramic to a reactive metal which reacts with the sacrificial ceramic to form an oxidized ceramic of the reactive metal and a reduced form of the ceramic. The resulting structure comprises the open-cell ceramic foam of the oxidized ceramic compound of the reactive metal with continuous ceramic cell walls and the continuous intracellular volume between the ceramic cell walls having a metallic reaction product therein. The metallic reaction product may be the same as the desired bonding-element first-portion metal. If not, the metallic reaction product may be replaced with the bonding-element first-portion metal.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.